Lookin at that diagram, it would appear they have the dams there to help slown down the smaller radius runners so they would be about the same speed as the larger radius runners. So why not on all four smaller radius runners? Maybe my thinkin is wrong. lol
My uneducated guess would be maybe it’s the way the order of intake draw of each cylinder happens, how the mixture is drawn which just causes those two runners to need them?? No idea really! Someone here knows for sure